Method for the vertical continuous casting of a steel strip

ABSTRACT

In a method for vertical continuous casting of a steel band, a strand ( 11 ) of a parallelogram-like cross section is cast at first in a revolving chill-mold ( 2 ) and is thereafter transferred from said initial cross section with completely solidified longitudinal edges ( 10 ) and liquid core into a band ( 1 ) with plane-parallel cross section, this occurring in such a way that an already solidified shell ( 9 ) of the strand ( 11 ) which becomes increasingly thicker by cooling is increasingly compressed in the casting direction in a forming device ( 4 ) without upsetting deformation of the completely solidified longitudinal edges ( 10 ). In order to provide advantageous conditions it is proposed that after the compression into a band ( 1 ) the strand ( 11 ) with a still liquid core ( 16 ) is guided during the complete solidification of the core merely in a forming gap with a constant width corresponding to the thickness of the completely solidified longitudinal edges ( 10 ) and is calibrated in this process.

FIELD OF THE INVENTION

[0001] The invention relates to a method for vertical continuous castingof a steel band, with a strand of a parallelogram-like cross sectionbeing cast at first in a revolving chill-mold and being thereaftertransferred from said initial cross section with completely solidifiedlongitudinal edges and liquid core into a band with plane-parallel crosssection, this occurring in such a way that an already solidified shellof the strand which becomes increasingly thicker by cooling isincreasingly compressed in the casting direction in a forming devicewithout upsetting deformation of the completely solidified longitudinaledges.

DESCRIPTION OF THE PRIOR ART

[0002] In the vertical continuous casting of steel bands it is known (EP0 329 639 B1) to cool the strand which is cast with a parallelogram-likecross section in a revolving chill-mold while maintaining the crosssection until a rigid shell which is already completely solidified isformed in the region of the longitudinal edges of the strand as a resultof the parallelogram-like cross section before the strand is furthercooled and is formed progressively with increasing solidification into aflat preliminary band and is compressed. The deformation of the strandwhich is cast with a parallelogram-like cross section into aplane-parallel preliminary band thus only occurs after the formation ofa sufficiently thick shell. The transfer of the parallelogram-like crosssection of the strand into a plane-parallel cross section occurs in aforming device which is composed of several longitudinal beams which aresituated opposite of each other with respect to the band and which formbetween themselves a forming gap with a parallelogram-like inlet crosssection and a plane-parallel outlet cross section, namely with the helpof section rollers which are held in the longitudinal beams. Since thelongitudinal beams are held in a swivelable fashion on the inlet sideand are subjected to pressurization within the terms of mutual pivoting,one obtains—under the precondition of an even cooling—a thickness forthe preliminary band completely solidified in the region of the formingdevice when the shells are pressed against each other, which thicknessdepends on the pass-through speed of the strand through the chill-moldand the forming device. The extent of the solidification determining theband thickness depends on the cooling duration which is a function ofthe pass-through speed of the band through the chill-mold and theforming device. This means that it is necessary to ensure a constantcasting speed for a uniform, even band thickness.

[0003] In view of the cooperation with an upstream steel works and thepossible direct further processing of the preliminary band in asubsequent rolling mill, a constant casting speed has operationaldisadvantages because circumstances can occur both in the steel works aswell as the rolling mill which require a change in the casting speed orthe casting output. Apart from that, temperature fluctuations in thesteel melt must be expected, which also entails fluctuations in thethickness of the preliminary band.

[0004] In order to simplify the reduction in thickness of continuouslycast steel bands it is known (DE 41 35 214 A1) to carry out a rollingdeformation of the cast strand before the liquid core is completelysolidified. Said rolling deformation requires a shell which is alreadysolidified in sufficient thickness, but which is compressed undercrushing of the liquid core by the rollers without upsettingdeformation, but is upset in the edge region. Said upsetting deformationon the edge is inevitably linked to an extension or lateral bulging,leading to wavy edges of the cast and thus reduced steel band or totensions and a likelihood of cracks in the edge region. Nothing ischanged in respect to this by a deformation-free guidance of the bandbetween guide rollers with a constant gap, which guidance is providedsubsequently to the reduction of the thickness and is provided for thecomplete solidification of the core.

SUMMARY OF THE INVENTION

[0005] The invention is thus based on the object of avoiding thesedisadvantages and to provide a method of the kind mentioned above insuch away that despite any occurring changes concerning the castingspeed and the temperature of the liquid steel it is possible to ensure aconstant band thickness without any likelihood of wavy edges orformation of cracks.

[0006] This object is achieved by the invention in such a way that afterthe compression into a band the strand with a still liquid core isguided during the complete solidification of the core merely in aforming gap with a constant width corresponding to the thickness of thecompletely solidified longitudinal edges and is calibrated in thisprocess.

[0007] Since for the purpose of the calibration of the band the bringingtogether of the shells is ended before the mutually opposite shellstouch each other and the thickness of the forming gap is kept constantduring the complete solidification of the thus remaining liquid coredepending on the cooling-induced shrinkages, the definite band thicknessis determined by the dimensions of the forming gap and not by thecasting speed. The ferrostatic pressure acting in the region of theremaining liquid core ensures that the shells rest on the formingelements predetermining the geometry of the gap. Different thicknessesof the liquid core arising from different casting speeds can thereforenot lead to any different band thicknesses as long as it is ensured thatthe core solidifies completely in the region of the constant thicknessof the forming gap. The relevant aspect is that in the edge region noupsetting deformation of the band can occur in connection with anextension. This is ensured in such a way that the forming gap has awidth corresponding to the thickness of the completely solidifiedlongitudinal edges. Since the shell of the strand is transferred afterthe casting in a revolving chill-mold in a forming device from aparallelogram-like cross section to a plane-parallel cross sectionwithout upsetting the completely solidified longitudinal edges of theshell, said completely solidified longitudinal edges of theparallelogram-like cross section also determine the later thickness ofthe completely solidified band, so that there is no upsettingdeformation of the longitudinal edge linked to an extension before theband is fully completely solidified.

[0008] Due to the complete solidification of the core without astretching effect on the band, a cast structure can substantially beexpected. After the complete solidification of the band the thickness ofthe band can be reduced slightly by simultaneous stretching preferablyby means of force- and path-controlled rollers, leading to a respectiveimprovement in the structure.

[0009] In order to enable the supply with especially thin preliminarybands for further processing in a rolling mill, the completelysolidified band can be additionally reduced in its thickness bycontinuous rolling after the calibration, thus considerably reducing theamount of rolling work in the rolling mill.

[0010] For the purpose of performing the calibration in accordance withthe invention of the continuously cast steel band it is possible toassume a continuous casting plant which consists of a chill-moldrevolving together with the strand to be cast and having a forming gapcross section in the shape of a parallelogram, and of a downstreamforming device with several rollers which are opposite of each otherwith respect to the strand and form between themselves a forming gapwith a parallelogram-like inlet cross section and a plane-paralleloutlet cross section. It needs to be ensured that a calibration devicewith a predetermined forming gap progress is provided adjacent to theforming device, which gap progress comprises a section with constantforming gap thickness at least on the inlet side, so that the completesolidification of the band occurs in the section of the forming gap withconstant thickness. Depending on the casting speed the point of completesolidification of the band will occur along the forming gap. In the caseof reduced casting speed, it is therefore possible to expect thecomplete solidification of the band in an initial section (i.e. in theupper region) and at an increased casting speed in an end section, i.e.in the lower region of the forming gap of the calibrating device.

[0011] Advantageous conditions are obtained when the calibrating devicecomprises calibrating rollers which delimit the forming gap and can beadvanced to set the progress of the forming gap. Not only the thicknessof the band can be determined in an advantageous fashion during itscomplete solidification between the calibrating rollers, but also anadvance can be achieved when said calibrating rollers are driven. Due tothe progressed solidification of the band it is not necessary to providea continuous band guidance, so that cooling liquid can be applied to theband between the calibrating rollers.

[0012] After the solidification of the liquid residual core it ispossible to perform a reduction in the thickness via the remainingcalibrating rollers on the outlet side, e.g. in the magnitude of 1 to5%, for the purpose of improving the structure, requiring a respectivelypreset progress of the forming gap.

[0013] The calibrating device can also reach far into the casting arc,allowing a reduction of the overall height.

[0014] In order to achieve an additional higher reduction in thethickness of the band, a reducing frame can be provided on the outletside of the calibrating device which can be used to supply comparativelythin preliminary bands to a connected rolling mill.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The method in accordance with the invention is explained below incloser detail by reference to the enclosed drawings, wherein:

[0016]FIG. 1 shows in a schematic longitudinal view a continuous castingplant in accordance with the invention for continuous casting;

[0017]FIG. 2 shows a simplified cross-sectional view of the calibratingdevice on an enlarged scale;

[0018] FIGS. 3 to 5 show the relevant changes in cross section of astrand cast according to the state of the art during its deformationinto a band, and

[0019] FIGS. 6 to 9 show a representation corresponding to FIGS. 3 to 5of the changes in cross section of a strand case in accordance with theinvention during its deformation into a band.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] According to FIG. 1, the illustrated continuous casting plant forthe vertical continuous casting of a steel band 1 comprises a revolvingchill-mold 2, a forming device 4 which is adjacent thereto via a strandguidance means 3, and a calibrating device 5 from which the band 1emerges in a casting arc 6 in order to be deflected by a vertical to ahorizontal progress. The calibrating device 5 can extend at least partlyinto the region of the casting arc 6, allowing for a lower overallheight.

[0021] The revolving chill-mold 2 consists of two mutually opposite,continuously revolving plate chains 7 which enclose a constant forminggap between themselves and into which opens a casting pipe 8 attached toa casting container. The plates of the plate chain 7 which are mutuallyassociated in pairs form a forming gap with a parallelogram-like crosssection, so that the liquid steel cast into the forming gap of therevolving chill-mold 2 through the casting pipe 8 is chilled in theregion of the plate chains 7 and, with progressing chilling, forms anincreasingly thicker solidified shell 8 which is completely solidifiedin the region of the longitudinal edges 10 as a result of the strandthickness decreasing towards the longitudinal edges 10, as is shown inFIGS. 3 and 4 as well as 6 and 7 which show the strand 11 at first inthe region of the revolving chill-mold 2 with a still thin shell 9 andthen with an increasing shell thickness. As a result of the completelysolidified longitudinal edges 10, a further guidance of the cast strand11 via the strand guidance means 3 to the forming device 4 is easilypossible, the forming gap of which gradually changes from theparallelogram-like inlet cross section to a plane-parallel outlet crosssection. For this purpose the forming device 4 comprises severallongitudinal beams 12 which are mutually opposite relative to the strand11, form a forming gap between themselves, are held swivelable on theinlet side about axes 13 and are pressurized via a pressure cylinders 14within the terms of a mutual swiveling. The forming gap is delimited bysection rollers 15.

[0022] As is shown in FIG. 5, the shells 9 are guided in this formingdevice 4 upwardly against each other under displacement of the liquidcore in accordance with the state of the art until they are pressedagainst each other and form a completely solidified band whose thicknessdepends on the casing speed or the temperature of the steel melt underconstant chilling conditions. In contrast thereto, the shells 9 are ledtogether in the forming device 4 only into a plane-parallel crosssection with a still liquid core 16, as is shown in FIG. 8. Notice mustbe taken that the longitudinal edges 10 are not subjected to anyupsetting deformation linked to the extension. The completesolidification of the liquid core 16 only occurs in the calibratingdevice 5 which at least on the inlet side has a forming gap of constantthickness, in the region of which the liquid core 16 solidifiescompletely until a band 1 of constant thickness is obtained according toFIG. 9. In this connection it needs to be considered that as a result ofthe ferrostatic pressure in the region of the liquid core 16 the shells9 are pressed against the calibrating device 5, so that thepredetermined forming gap width determines the band thicknessirrespective of the casting speed or the thickness of the liquid core16. The forming gap width needs to be chosen according to the thicknessof the completely solidified longitudinal edges 10, so that anyextension of the band 1 on the edge side is prevented.

[0023] The cooling, the pass-through speed of the strand 11 through therevolving chill-mold 2 and the forming device 4 as well as the length ofthe chill-mold 2 and the forming device 4 are adjusted to each other insuch a way that the strand 11 still has a liquid core 16 on emergingfrom the forming device 4, because the complete solidification of theband 1 should only occur in the calibrating device 5. The calibratingrollers 17 of the calibrating device 5 are set via actuating cylinders18 to a forming gap progress predetermined for the calibrating process.Since a considerable ferrostatic pressure prevails in the region of theliquid core 16, the shells 9 of the band 1 are pressed outwardly againstthe calibrating rollers 17 in the region of the calibrating device 5,thus ensuring the desired calibrating effect. The respective thicknessof the liquid core 16 can vary upon the entrance of the band 1 into thecalibrating device 5.

[0024] Since a plane-parallel band cross section is already assumed inthe calibrating device 5, the calibrating rollers 17 run through overthe band width, as is shown in FIG. 2. The schematically indicated drive19 of the calibrating rollers 17 supports band conveyance and is alsosuitable for a low rolling output following the complete solidificationof the liquid core 16, so that after the complete solidification of theband 1 the calibrating rollers 17 can be used for a slight reduction inthe thickness under a stretching effect. In order to enable theperformance of stronger reductions in thickness, the calibrating device5 can be provided downstream with a rolling frame 20. Said rolling frame20 can also be provided after the casting arc 6, as is indicated withthe dot-dash line in FIG. 1.

1. A method for vertical continuous casting of a steel band, with astrand (11) of a parallelogram-like cross section being cast at first ina revolving chill-mold (2) and being thereafter transferred from saidinitial cross section with completely solidified longitudinal edges (10)and liquid core into a band (1) with plane-parallel cross section, thisoccurring in such a way that an already solidified shell (9) of thestrand (11) which becomes increasingly thicker by cooling isincreasingly compressed in the casting direction in a forming device (4)without upsetting deformation of the completely solidified longitudinaledges (10), characterized in that after the compression into a band (1)the strand (11) with a still liquid core (16) is guided during thecomplete solidification of the core merely in a forming gap with aconstant width corresponding to the thickness of the completelysolidified longitudinal edges (10) and is calibrated in this process. 2.A method as claimed in claim 1, characterized in that the band (1) isreduced in its thickness directly after the complete solidificationunder simultaneous stretching.
 3. A method as claimed in claim 1,characterized in that the completely solidified band (1) is reduced inits thickness after the calibration by rollers.
 4. A continuous castingplant for performing the method as claimed in claim 1, consisting of achill-mold (2) revolving with the strand (11) to be cast with aparallelogram-like forming gap cross section and of a downstream formingdevice (4) with several rollers (15) which are opposite of each otherwith respect to the strand (11) and form between themselves a forminggap with a parallelogram-like inlet cross section and a plane-paralleloutlet cross section, characterized in that a calibrating device (5)with a predetermined forming gap progress is provided downstream of theforming device (4), which forming gap comprises a section with aconstant forming gap width at least on the inlet side.
 5. A continuouscasting plant as claimed in claim 4, characterized in that thecalibrating device (5) comprises calibrating rollers (17) which delimitthe forming gap and which can be adjusted for setting the forming gapprogress.
 6. A continuous casting plant as claimed in claim 5,characterized in that the calibrating device (5) is situated at leastpartly in the region of the casting arc (6).
 7. A continuous castingplant as claimed in claim 4, characterized in that a rolling frame (20)is provided on the outlet side of the calibrating device (5).